In the manufacture of molded records, such as conventional audio records or the more recently developed high density information discs, a plastic material is molded between a pair of metal plates referred to as stampers. The stampers have defined in their molding surface a spiral information track which contains a mirror image of the surface relief pattern which corresponds to the information desired to be molded into the surface of the record.
The stampers used to mold the records are the end product of a multistep replication process known as matrixing. The first step in the matrixing process is to record on magnetic tape or the like the program information desired to be molded into the record. The tape is then used to control a tool which cuts a spiral information track into a flat disc which is referred to as a recording substrate. The recording substrate is generally a metal plate which has a layer of material such as wax, lacquer or a metal such as copper formed on its surface into which the information track is cut.
The recorded substrate having the information track defined in the surface is thereafter replicated. In the first replication step a metal such as nickel is electrodeposited on the surface of the recording substrate until a sufficiently thick layer is deposited so as to make a self-sustaining port, usually about 0.015 inches (0.04 cm) in thickness. The resulting electroformed part, which is referred to as a master, is removed from the surface of the recording substrate. The master which is obtained is essentially a mirror image of the surface of the recording substrate. The master is then in turn replicated by electroforming a metal such as nickel to form a mold. A number of molds are generally formed on a given master. The molds which are obtained are mirror images of the master and are positive copies of the original recording substrate. The molds in turn are themselves replicated to form stampers. The stampers are generally somewhat thinner than the masters and molds being generally about 0.008 inches (0.02 cm) in thickness. If the matrixing process is conducted correctly, the stampers will have defined in their surface a mirror image of the surface relief pattern which is desired to be molded into the record.
In the matrixing process, parts such as the masters and molds are initially formed as replicas and are then themselves used as matrixes, that is a part on which a replica is formed. Since the present invention is generally useful in the matrixing process without specific regard to whether a part is a recording substrate, master, mold or stamper, the term matrix will be used hereinafter to refer to a part which is replicated and the term replica will be used hereinafter to refer to a part which is electroformed on a matrix.
The matrixing process is a highly complex procedure which must be carefully conducted in order to consistently produce satisfactory products. One of the most important considerations in the matrixing process is to prevent the matrix and replica from being stained. Another consideration is to prevent burnout of the parts during the electroforming. One of the principle causes of staining and burnouts has been found to be leakage of electrolyte between the matrix and the replica during electroforming. Another major problem area in the matrixing process is premature separation of the replicas from the matrixes during and immediately after electroforming. This is related in part to the electrolyte leakage problem as it is caused to a large extent by poor sealing of the replica to the surface of the matrix.
The problem of poor sealing of the replica to the matrix is an extremely difficult problem to overcome in that certain required matrixing procedures are in fact a cause of the problem. For example, in the matrixing process the matrixes must be passivated so as to prevent adhesion and possible alloying of the metal which is electrodeposited to form the replica. The matrix and resulting replica are also deliberately designed so they do not have any undercutting or the like which would mechanically interlock the parts and prevent separation after electroforming. In addition, the outer edges as well as the center portions of the stampers are specifically designed to be as smooth as possible, almost mirrorlike, so as to promote the release of molded plastic records from the stampers. Even the recorded areas of the stampers, especially stampers used for the high density information discs, are essentially smooth because of the relative closeness and shallowness of the information tracks in the recorded program area. For example, on a high density information disc there are about 10,000 grooves to the inch, which grooves are only about 19 microinches in depth, so that the surface is in effect smooth.
Various suggestions have been made to reduce or eliminate electrolyte leakage and premature separation of replicas and matrixes. Once such suggestion was to preplate the matrixes with the outer circumferential edge exposed so as to cause preplating over and around the outer edge of the matrix. The preplating over the outer edge of the matrix was intended to form a seal about the outer edge of the matrix and also to form a grip to hold the replica in place on the matrix during and after the electroforming operation. This technique has not proved to be satisfactory. It had limited success in reducing electrolyte leakage and premature separation and further added a separate, time-consuming step to the matrixing process. This technique also caused problems in the separation of the replicas from the matrixes because the extra metal on the outer edges has to be removed either by being cut off or ground off.
It was also suggested to knurl the unrecorded outer edges and also the center of the matrixes so as to provide a grip for the matrix. Knurling, however, did not prove to be a satisfactory solution. It was found that the knurling had to be forced into the replica for a substantial depth in order to have an effect in holding a replica onto the surface of a matrix. The amount and depth of the knurling required resulted in a considerable amounf of metal being displaced in the knurling process which in turn caused distortion of the information track in the recorded area of the replica. A further problem encountered with knurling was that if the knurling was sufficiently deep, it often resulted in deep lines or raised ridges at the outer edges of the molded record which was highly undesirable with regard to using that portion for the lead-in track on the record.
Recently a suggestion was made by John J. Prusak in U.S. patent application Ser. No. 325,955, filed Nov. 30, 1981 which is entitled, "Matrixing Processes for the Manufacture of Molded Records." Prusak suggested the use of a texturized band around the outer diameter and the inner diameter of the record in order to form a seal which would be replicated through the entire matrixing process. The technique suggested by Prusak has proven to be a substantial improvement over the methods heretofore suggested in the art. The Prusak method, however, has not proven to be completely satisfactory in that occassionally electrolyte leaks occur and premature separations are encountered, particularly if the operators are not skilled in the use of the technique disclosed by Prusak.
It would be highly advantageous if an apparatus and method could be provided which would effectively provide an electrolyte seal to prevent leakage of electrolyte between replicas and matrixes as well as prevent premature separation of the replicas from the surface of the matrixes.